Photoflashunit for photographic apparatus

ABSTRACT

The photoflashbulb is fired by the voltage generated by a piezoelectric crystal when the crystal is struck by a hammer released by a shutter when the latter is operated. Shunted across the crystal and bulb is a photoresistor, the resistance of which is sufficiently small, when scene brightness is adequate to permit taking pictures without the flash of the bulb, to dampen the crystal so that the voltage generated is too small to fire the bulb. In a second embodiment the conductivity of a transistor, connected in shunt with the crystal, is controlled by the photoresistor, the transistor being conductive whenever the scene brightness is adequate.

United States Wagner et a1.

[ PHOTOFLASHUNIT FOR PHOTOGRAPHIC APPARATUS [75] Inventors: Karl Wagner, Ottobrunn; Josef Ganser, Munich, both of Germany [73] Assignee: Agfa-Gevaert Aktiengesellschaft,

.Leverkusen, Germany 22 Filed: Nov. 12,1970

21 Appl. No.2 88,929

[30] Foreign Application Priority Data Nov. 12, 1969 Germany ..P 19 56 764.7

[52] U.S.Cl. ..95/l1 L, 95/10 C, 240/13 [51] Int. Cl. ..G03b 15/04, HOlh 57/00 [58] Field of Search ..95/11 R, 11.5 R,

3,232,191 Sherwood ..95/l0 C Apr. 10, 1973 Primary Examiner-Robert P. Greiner Attorney-Michael S. Striker [57] ABSTRACT The photoflashbulb is fired by the voltage generated by a piezoelectric crystal when the crystal is struck by a hammer released by a shutter when the latter is operated. Shunted across the crystal and bulb is a photoresistor, the resistance of which is sufficiently small, when scene brightness is adequate to permit taking pictures without the flash of the bulb, to dampen the crystal so that the voltage generated is too small to tire the bulb. in a second embodiment the conductivity of a transistor, connected in shunt with the crystal, is controlled by the photoresistor, the transistor being conductive whenever the scene brightness is adequate.

18 Claims, 2 Drawing Figures PAIENTEMPRI I 3,726,195

IN V EN TOR PHOTOFLASHUNIT FOR PHOTOGRAPHIC APPARATUS BACKGROUND OF THE INVENTION switching arrangement. As a rule, the photographer must operate this switching arrangement by hand when he requires the additional illumination provided by the bulb.

SUMMARY OF THE INVENTION An object of the invention is a photoflashunit that automatically permits firing of the photoflashbulb only when the scene brightness is sufficiently low to require the additional illumination providedby the bulb.

The invention consists essentially of a photoflashbulb having a predetermined resistance before the bulb is flashed, means connected to the bulb for producing a voltage to fire the latter, and a light sensitive control arrangement exposed at least in part to scene light and connected in parallel with the bulb, this light sensitive control arrangement having a resistance that increases as the scene light decreases, the resistance being smaller than the predetermined resistance of the photoflashbulb for any scene brightness equal to or exceeding a predetermined scene brightness and greater than this predetermined resistance for any scene brightness smaller than the predetermined scene brightness, so that the photoflashbulb is flashed only when the scene brightness is below the determined scene brightness.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from-the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing one embodiment of the invention; and I FIG. 2 is a circuit diagram showing a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, a piezoelectric crystal 1 generates a high voltage pulse when suddenly mechanically stressed The crystal 1 is stressed by a hammer 3, biassed by a spring 2, the hammer being pivoted about a pin 4.

A spring biassed tensioning and release catch 5 is connected, in a mannernot shown, with a release and a shutter actuating ring. The release tensions the catch 5, which, for example, is released directly, or through a lever, by the shutter actuating ring. Shunted across the crystal 1 is a high resistance photoflashbulb 6, which has two specially designed electrodes 7 and 8. Connected parallel to the photoflashbulb 6 is a photoresistor 9, which is exposed to scene light and has a sufficiently high breakdown voltage. When the scene light is insufficient to take a picture without a photoflashbulb, the resistance of the photoresistor 9 is high with respect to that of the bulb 6; but when the scene light is sufficient, the resistance of the photoresistor is low with respect to that of the bulb.

When the camera shutter is cocked, the hammer 3, n a manner not shown, is moved to its tensioned position, shown in FIG. 1; and when the shutter is released, the catch 5 pivots in the direction of the arrow A, thereby freeing the hammer 3 of which the head 3a strikes the piezoelectric crystal 1, which is held fixedly in the housing of the camera. The sudden elastic deformation of the piezoelectric crystal causes a voltage pulse of between approximately 3,000 nd 5,000 volts to appear across the contact electrodes la and lb. If the scene brightness is insufficient for taking a picture without a flash, the resistance of the photoresistor 9 is greater than the resistance of the photoflashbulb 6, the latter resistance being approximately 10 ohms. The voltage at the electrodes 7 and 8 of the bulb 6 is sufficient to cause the latter to flash. The photoflashbulb 6, can, for example, be one bulb of a flashcube. If the scene brightness is sufficient, the resistance of the photoresistor 9 is less than that of the bulb 6. Consequently, when the hammer 3 strikes the piezoelectric crystal 1, the voltage pulse is too small to flash the bulb 6.

The reference numerals for the two embodiments shown in FIGS. 1 and 2 are the same for all parts having the same function in both embodiments.

With reference to the embodiment shown in FIG. 2, there is shunted across the piezoelectric crystal 1 and the photoflashbulb 6 a voltage divider consisting of a photoresistor 10 and a fixed resistor 11, the junction between these two components being connected to the base of a transistor 12, which latter acts as an electronic switch. Resistors 13 and 14 are respectively connected to the collector and the emitter of this transistor. A resistor 15 shunts the emitter collector path of the transistor. The resistances of the resistors 13 and 14 are smaller than the resistance of the bulb 6,

. whereas the resistance of the resistor 15 is greater than that of the bulb.

If the scene brightness is insufficient, the bulb 6 is flashed in the same manner as in the embodiment shown in FIG. 1, since the effective resistance of the light-sensitive control arrangement composed of the components 10, ll, 12, 13, 14, and 15, is greater than that of the bulb 6. The voltage produced by the piezoelectric crystal 1 is sufficient to flash the bulb. When the scene brightness is insufficient, the transistor 12 is non-conductive because its base is insufficiently negative with respect to the emitter. Consequently, it is the sum of the resistances 13, 14, and 15, shunted across the bulb 6, that determine the current applied to the electrodes 7 and 8.

However, if the brightness is sufficient to permit taking a picture without a flash, the resistance f the photoresistor 10 is such that the transistor 12 is turned on. As a result, it is substantially the series connection of the resistors 13 and 14 that determines the current applied to the electrodes 7 and 8. Because the crystal 1 is highly damped the voltage across the latter is too small to flash the bulb 6.

In accordance with the invention, a thyristor can be substituted for the transistor 12.

The invention insures that although a photoflashbulb is inserted the bulb flashes only when the scene light is too low to enable adequate exposure without the illumination of the flash. The light sensitive control arrangement in both embodiments (the photoresistor 9 in FIG. 1 and, in FIG. 2, the photoresistor 10 and transistor 12 and associated resistors) is connected at all times to the piezoelectric crystal and also to the photoflashbulb when the latter is inserted in the photoflashunit. The hammer is released to strike the crystal every time that the shutter is released.

The invention also has the advantages that it is simply constructed and reliable in operation. A further advantage of the invention is that it operates without switching contacts.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits and constructions, differing from the types described above.

While the invention has been illustrated and described as embodied in photoflashunit for photographic apparatus, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such'adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected.

by Letters Patent is set forth in the appended claims.

We claim:

1. For use with a photographic apparatus, a photoflash unit comprising in combination a flashbulb circuit branch having two end terminals and includinga flashbulb; energy supply means connected directly across said end terminals in parallel with said flashbulb circuit branch and including a piezoelectric crystal operative for generating a surge of electrical energy;

and scene-light-responsive impedance means connected directly across said end terminals in parallel with said flashbulb circuit branch and having an impedance which decreases in response to increasing scene brightness, for so controlling the magnitude of the current flowing out of said supply means and through said impedance means and the voltage generated across said supply means as to prevent firing of said flashbulb when the detected scene brightness is greater than a predetermined scene brightness.

2. A photoflash unit as defined in claim 1, wherein said impedance means comprises electronic switch means having an input and a light-sensitive element connected to said input for controlling the conductivity of said switch means as a-function of scene light.

3. A photoflashunit as defined in claim 2, wherein said electronic switch means is a thyristor 4. A photoflashunit as defined in claim 2, wherein said switch means is a transistor, said input being the base of said transistor.

5. A photoflashunit as defined in claim 4, wherein said arrangement further includes a first resistor connected in series with a light sensitive element, said series connected first resistor and light sensitive element being shunted across said photoflashbulb, and the base of said transistor being connected to the junction between said first resistor and light sensitive element so that said transistor is conductive only when the scene brightness is equal to or exceeds said predetermined scene brightness.

6. A photoflashunit as defined in claim 5, said arrangement further including second and third resistors, respectively connected to the collector and emitter of said transistor, and a fourth resistor shunted across the collector and emitter of said transistor, said second, third and fourth resistors forming a series pa'th connected in parallel with said photoflashbulb.

7. A photoflashunit as defined in claim 6, wherein the resistance of said fourth resistor is greater, nd the resistances of said second and third resistors are smaller, than said predetermined resistance of said photoflashbulb.

8. A photoflashunit as defined in claim 7, herein said light sensitive element is photoresistor means exposed to scene light.

9. A photoflashunit as defined in claim 8, wherein said photoflashbulb has two electrodes, and said series connected second, third, and fourth resistors and said series connected photoresistor means and first resistor are shunted across said two electrodes.

10. A photoflash unit as defined in claim 1, herein said impedance means has a light-dependent net resistance greater than the resistance of said flashbulb circuit branch when the detected scene brightness is lower than said predetermined brightness, and lower than the resistance of said flashbulb circuit branch when the detected brightness is greater than said predetermined brightness.

11. A photoflash unit as defined in claim 1, wherein said predetermined brightness corresponds to the minimum brightness at which the film can be adequately exposed without light from said flashbulb.

12. A photoflashunit as defined in claim l, further including hammer means arranged to stroke said crystal 13. A photoflashunit as defined in claim 1, wherein the photoflashunit is part of the photographic apparatus.

14. A unit as defined in claim 1, wherein said impedance means is permanently electrically connected with said bulb, and wherein said bulb is permanently electrically connected in parallel with said piezoelectric crystal.

15. A unit as defined in claim 14, wherein said impedance means consists of a photosensitive element.

16. A unit as defined in claim 15, wherein said impedance means consists of a photoresistor.

17. A photoflashunit as defined in claim 16, herein said flashbulb has two electrodes, and said photoresistor is shunted directly across said two electrodes.

18. For use with a photographic apparatus, a photoflash unit comprising in combination, a flashbulb circuit branch having two end terminals and including a flashbulb; energy supply means connected directly across said end terminals in parallel with said flashbulb circuit branch and including a piezoelectric crystal the voltage generated across said supply means as to prevent firing of said flashbulb when the detected scene brightness is greater than a predetermined scene brightness. 

1. For use with a photographic apparatus, a photoflash unit comprising in combination a flashbulb circuit branch having two end terminals and including a flashbulb; energy supply means connected directly across said end terminals in parallel with said flashbulb circuit branch and including a piezoelectric crystal operative for generating a surge of electrical energy; and scene-light-responsive impedance means connected directly across said end terminals in parallel with said flashbulb circuit branch and having an impedance which decreases in response to increasing scene brightness, for so controlling the magnitude of the current flowing out of said supply means and through said impedance means and the voltage generated across said supply means as to prevent firing of said flashbulb when the detected scene brightness is greater than a predetermined scene brightness.
 2. A photoflash unit as defined in claim 1, wherein said impedance means comprises electronic switch means having an input and a light-sensitive element connected to said input for controlling the conductivity of said switch means as a function of scene light.
 3. A photoflashunit as defined in claim 2, wherein said electronic switch means is a thyristor
 4. A photoflashunit as defined in claim 2, wherein said switch means is a transistor, said input being the base of said transistor.
 5. A photoflashunit as defined in claim 4, wherein said arrangement further includes a first resistor connected in series with a light sensitive element, said series connected first resistor and light sensitive element being shunted across said photoflashbulb, and the base of said transistor being connected to the junction between said first resistor and light sensitive element so that said transistor is conductive only when the scene brightness is equal to or exceeds said predetermined scene brightness.
 6. A photoflashunit as defined in claim 5, said arrangement further including second and third resistors, respectively connected to the collector and emitter of said transistor, and a fourth resistor shunted across the collector and emitter of said transistor, said second, third and fourth resistors forming a series path connected in parallel with said photoflashbulb.
 7. A photoflashunit as defined in claim 6, wherein the resistance of said fourth reSistor is greater, nd the resistances of said second and third resistors are smaller, than said predetermined resistance of said photoflashbulb.
 8. A photoflashunit as defined in claim 7, herein said light sensitive element is photoresistor means exposed to scene light.
 9. A photoflashunit as defined in claim 8, wherein said photoflashbulb has two electrodes, and said series connected second, third, and fourth resistors and said series connected photoresistor means and first resistor are shunted across said two electrodes.
 10. A photoflash unit as defined in claim 1, herein said impedance means has a light-dependent net resistance greater than the resistance of said flashbulb circuit branch when the detected scene brightness is lower than said predetermined brightness, and lower than the resistance of said flashbulb circuit branch when the detected brightness is greater than said predetermined brightness.
 11. A photoflash unit as defined in claim 1, wherein said predetermined brightness corresponds to the minimum brightness at which the film can be adequately exposed without light from said flashbulb.
 12. A photoflashunit as defined in claim 1, further including hammer means arranged to stroke said crystal
 13. A photoflashunit as defined in claim 1, wherein the photoflashunit is part of the photographic apparatus.
 14. A unit as defined in claim 1, wherein said impedance means is permanently electrically connected with said bulb, and wherein said bulb is permanently electrically connected in parallel with said piezoelectric crystal.
 15. A unit as defined in claim 14, wherein said impedance means consists of a photosensitive element.
 16. A unit as defined in claim 15, wherein said impedance means consists of a photoresistor.
 17. A photoflashunit as defined in claim 16, herein said flashbulb has two electrodes, and said photoresistor is shunted directly across said two electrodes.
 18. For use with a photographic apparatus, a photoflash unit comprising in combination, a flashbulb circuit branch having two end terminals and including a flashbulb; energy supply means connected directly across said end terminals in parallel with said flashbulb circuit branch and including a piezoelectric crystal operative for generating a surge of electrical energy; and scene-light-responsive impedance means connected directly across said end terminals in parallel with said flashbulb circuit branch and having an impedance which decreases in response to increasing scene brightness, for so controlling the magnitude of the voltage generated across said supply means as to prevent firing of said flashbulb when the detected scene brightness is greater than a predetermined scene brightness. 